1. Field of the Invention
The present invention relates to an image heating apparatus, adapted for use as a fixing apparatus to be incorporated in an image forming apparatus for example of electrophotographic process such as a copying apparatus or a printer.
2. Related Background Art
In an image forming apparatus utilizing an electrophotographic process, such as a printer, a copying apparatus or a facsimile apparatus, there is widely known, for fixing an unfixed toner image formed on a recording material, a fixing apparatus of heat roller type utilizing contact heating of satisfactory heat efficiency and safety.
Also in recent years, from the standpoint of energy saving, attention is being paid to a fixing apparatus of film heating type, utilizing a heating through a fixing film of a low heat capacity, as a “quick starting” system of a higher heat transmission efficiency and a faster start-up of the apparatus in comparison with the fixing apparatus of heat roller type, and the fixing apparatus of such film heating type is proposed for example in Japanese Patent Application Laid-open No. H04-44075.
The fixing apparatus of film heating type includes a configuration of conveying a fixing film under a tension by means of an exclusive conveying roller and an idler roller, in cooperation with a pressure roller, and a configuration of driving a cylindrical fixing film by a conveying force of a pressure roller. The former configuration has an advantage of improving the conveying ability for the fixing film, while the latter has an advantage of simplifying the structure thereby realizing a fixing apparatus of a low cost.
As a specific example, FIG. 10 shows a schematic lateral cross-sectional view of a fixing apparatus of film heating type (hereinafter referred to as a film fixing device) of the latter configuration driven by the pressure roller.
A stay 101 is elongated in a direction perpendicular to the plane of the drawing, and, a lower longitudinal face fixedly supports a ceramic heater (heat member) 108 prepared by forming a resistance heat member on a ceramic substrate. A heat-resistant cylindrical fixing film 102, constituting a heating rotary member, is loosely fitted on the stay 101 supporting the heat member 108. An elastic pressure roller 104, formed by heat-resistant rubber and constituting a pressurizing rotary member, forms a fixing nip portion N with the heat member 108 of the stay 101, through the fixing film 102. The pressure roller 104 is rotated counterclockwise as indicated by an arrow by an unillustrated driving system, whereby the fixing film 102 is driven around the stay 101, with an internal surface in a sliding motion in contact with the surface of the heat member 108 at the fixing nip portion N. A recording material P bearing an unfixed toner image T is introduced into the fixing nip portion N and is pinched and conveyed therein. Thus, the unfixed toner image T is fixed as a permanent image Ta on the recording material by a heat provided from the heat member 108 trough the fixing film 102 in the fixing nip portion N, and a pressure of the fixing nip portion N. The recording material emerging from the fixing nip portion N is further conveyed by discharge rollers 113 for discharging. For an appropriate temperature control of the heat member 108, a thermistor 107 for measuring the temperature of the heat member 108 is provided thereon. An arrowed direction with symbol “X” indicates a conveying direction of the recording material.
Together with the recent advancement in the computer industry, the printers are showing an increasing demand and are being used worldwide. As a result, there results a diversification of the recording material P in the type, thickness and surface of paper and also there are required a shorter first print time and a secure fixing property on the first print for responding to an increasing speed of the image forming apparatus, but a satisfactory fixing property has been attained by employing a higher fixing temperature or an improved thermal conductivity of the fixing film, thereby gradually increasing an instantaneous heat amount provided from the heat member to the recording material. Also in response to a requirement for a higher image quality of the user, printers with excellent dot reproducibility and gradation are being commercialized, and a higher image quality is also attained by employing a smaller toner particle size.
Under the aforementioned situation, various image defects may be generated also in a fixing step of the image forming process, but such defects have been avoided by various configurations.
One of such image defects is a toner offsetting. The offset means a phenomenon in which, when the recording material P bearing an unfixed toner image T passes through the fixing nip portion N, a part of the unfixed toner image T on the recording material P sticks onto the fixing film 102 serving as the heating rotary member and is re-transferred onto the recording material when the fixing film 102 after a turn comes into contact again with the recording material P thereby being fixed as a ghost image on the recording material.
The offset phenomenon includes one generated by a temperature factor and one generated by an electrostatic factor. The offset phenomenon generated by the temperature factor can be resolved by a temperature optimization, but that generated by the electrostatic factor is difficult to resolve.
In the following, there will be explained a mechanism of such offset phenomenon generated by an electrostatic charge (hereinafter referred to as electrostatic offset). When the recording material P enters the fixing nip portion N as schematically shown in FIG. 13 and in case the fixing film 102 is charged in a polarity opposite to the charging polarity of the toner constituting the unfixed toner image T and the surface of the pressure roller 104 is charged in a polarity same as the charging polarity of the toner constituting the unfixed toner image T, an electric field E1 in a direction indicated by arrows is generated in immediate front of the fixing nip portion, thereby exerting a force of separating a part of the toner of the unfixed toner image T from the recording material P. FIG. 13 shows, as an example, a case where the toner is charged negatively. Thus, a part of the unfixed toner image T on the recording material P loses a holding power thereon and is electrostatically deposited onto the fixing film 102. Such toner deposited on the fixing film 102 is, after a turn of the fixing film 102, re-transferred and then fixed on the recording material P as described above. Therefore, as schematically shown in FIG. 14, an offset pattern Ta′, formed as a ghost image of a normal image pattern Ta and at a behind position, corresponding to a turn of the fixing film, in the conveying direction of the recording material P, is formed on the recording material P.
For solving such electrostatic offset, Japanese Patent No. 2675886 proposes, in a fixing apparatus for fixing a toner image by pinching and conveying a recording material bearing an unfixed toner image between a pair of moving members, a configuration of applying a bias voltage to a moving member and grounding the other moving member or applying a bias voltage through a rectifying element. In the configuration disclosed in Japanese Patent No. 2675886, a conductive material is blended in an elastic layer of the pressure roller to reduce a resistance thereof, whereby an eventual charge on the surface of the pressure roller is dissipated through a metal core to the ground whereby the charging on the pressure roller can be suppressed.
Another image defect is a trailing phenomenon or a smear of the toner image.
When a recording material P enters the fixing nip portion N of a film fixing device, there may result a phenomenon that a part of the unfixed toner image T on the recording material P is scattered in a stripe shape toward a rear direction in the conveying direction X of the recording material. Such phenomenon is called a trailing or a smear.
Such trailing is a phenomenon in which, when the recording material P enters the fixing nip portion N as schematically shown in FIG. 11, a part T0 of the unfixed toner image T on the recording material P is scattered rearward in the conveying direction of the recording material, then shifted as T1 and heat fixed in the fixing nip portion N, thereby forming a scattered image as shown in FIG. 12, in which Ta indicates a fixed image of the unfixed toner image T and T1′ indicates a trailing portion formed by the aforementioned toner scattering.
The aforementioned tailing T1′ is assumed to be caused by water vapor, which is generated by an abrupt heating of moisture, contained in the recording material P, in the fixing nip portion N. Thus generated water vapor tends to escape in a direction indicated by a broken-lined arrow Y in FIG. 11, and blows off a part T0 of the unfixed toner image T on the recording material P, thereby causing such phenomenon.
Also an increase in the heat amount supplied for fixing for achieving a higher speed of the apparatus elevates the temperature of the fixing nip portion N. In such case the water vapor is generated in a larger amount and more strongly blows off the part T0 of the unfixed toner image T thereby facilitating the generation of the trailing T1′.
As the water vapor tends to escape in a direction opposite to the conveying direction a of the recording material P, the scattering phenomenon at the fixation is conspicuous in a lateral line image and also in case the line has a large width with a larger deposition amount of the toner on the recording material.
FIG. 15 schematically shows a configuration having countermeasures against the toner image offsetting and the toner image trailing in the film fixing device shown in FIG. 10. A metal core 104a of the pressure roller 104 is grounded through a rectifying element 112 and a power source 1 thereby preventing an accumulation of a charge of a polarity same as that of the toner, and a voltage of a polarity opposite to that of the toner is applied to the metal core 104a of the pressure roller 104 thereby injecting a charge of a polarity opposite to that of the toner to the surface of the pressure roller. Also a voltage of a polarity same as that of the toner is applied to the fixing film 102, from a power supply 2 through a feeding member 111. Such voltage applications exert, immediately in front of the fixing nip portion N, a force for holding the unfixed toner on the recording material P. Such force prevents generation of both the trailing and the electrostatic offset, particularly for the latter. Also the fixing discharge rollers 113 are formed by conductive members and grounded, whereby, when the recording material is pinched both by the fixing nip portion N and by the nip of the fixing discharge rollers 113, there is formed a conductive path of fixing film surface—recording material P—fixing discharge rollers 113. The voltage applied to the fixing film 102 induces a current from the fixing film surface to the fixing discharge rollers 113, and an electric field induced by such current exerts a toner supporting force, thereby principally suppressing the trailing phenomenon.
The prior technology explained in the foregoing sufficiently meets the requirements in the past, but a further improvement of performance is requested for the recent image forming apparatus of a higher speed.
For example, the film fixing device of the configuration shown in FIG. 15 in an actual image output operation (hereinafter referred to as printing) provided a satisfactory image without a trailing or an electrostatic offset in a low-speed range with a recording material conveying speed (hereinafter also referred to as a process speed) of 100 mm/sec or less. However, in a printing with a plain paper of a basis weight of 75 g/m2 in a speed range of a process speed of 150 mm/sec or higher, an image pattern offsetting may be generated within a width of 35 mm in a leading end portion of the recording material, and such offset became more conspicuous at a process speed of 200 mm/sec or higher.
In the configurations disclosed in Japanese Patent No. 2675886 and shown in FIG. 15, a conductive material is blended in the elastic layer of the pressure roller to reduce the resistance thereof, whereby an eventual charge on the surface of the pressure roller is dissipated through the metal core to the ground whereby the charging on the pressure roller is suppressed to prevent the electrostatic offset.
On the other hand, Japanese Patent Application Laid-open No. 2004-109175 proposes to suppress the electrostatic offset by a charge elimination of the surface of the pressure roller by a charge eliminating member.
However, it is found that the configuration of Japanese Patent Application Laid-open No. 2004-109175, despite of the presence of the charge eliminating member, cannot provide a sufficient charge eliminating effect, thus being unable to sufficiently suppress the electrostatic offset phenomenon.